Chromium(VI) salts are well known to be mutagens and carcinogens and to easily cross the cell membranes. Because they are powerful oxidizing agents, Cr(VI) reacts with intracellular materials to reduce to trivalent form, which binds DNA. This study was designed to investigate the interaction of calf thymus DNA with Cr(VI) and Cr(III) in aqueous solution at pH 6.5-7.5, using Cr(VI)/DNA(P) molar ratios (r) of 1:20 to 2:1 and Cr(III)/ DNA(P) molar ratios (r) of 1:80 to 1:2. UV-visible and Fourier transform infrared (FTIR) difference spectroscopic methods were used to determine the metal ionbinding sites, binding constants, and the effect of cation complexation on DNA secondary structure. Spectroscopic results showed no interaction of Cr(VI) with DNA at low anion concentrations (r ‫؍‬ 1:20 to 1:1), whereas some perturbations of DNA bases and backbone phosphate were observed at very high Cr ( Chromium(VI) salts are well known to be mutagens and carcinogens and to easily invade the insides of cells (1). Cr(VI) produced DNA cross-links in rat tissues in vivo (2) and in cultured cells in vitro (3, 4). Although Cr(VI) damaged nuclear DNA in whole cells, no reaction of Cr(VI) with isolated DNA occurred in vitro at physiological pH in the absence of a metabolizing system (5). The Cr(VI) that is taken up is considered to be reduced by glutathione, cysteine, or ascorbic acid into Cr(III) (6), and the resulting cation reacts with DNA to form Cr(III)-DNA adducts. Because Cr(III) is a final form of chromium within the cell, the interaction of Cr(III) with DNA may play crucial role in the carcinogenetic action of Cr(VI) salts.The conversion of B form into Z form in the purine-pyrimidine sequence of DNA has been considered to be a factor in the transcriptional activity of genes (7). Cr(III) is found to interact with the poly(dG-dC) at low concentration and change B form to Z form in the presence of ethanol (8). However, Cr(III) at high concentration causes DNA condensation, inhibiting the alteration of B to Z structure (8). Moreover, the study on the effect of Cr(III) on DNA replication with single-stranded DNA template and micromolar concentration of Cr(III) revealed that Cr(III) bound in a dose-dependent manner to the template DNA and prevents DNA replication (9). However, if the unbound chromium was removed from the system by gel filtration, the rate of DNA replication by polymerase I (Klenow fragment) on the chromium-bound template increased more than 6-fold relative to control. This increase was paralleled by as much as a 4-fold increase in processivity and a 2-fold decrease in replication fidelity. When the concentration of Cr(III) increased further, DNA-DNA cross-links occurred to inhibit the polymerase activity. Trivalent chromium can bind purified DNA and form lesions capable of obstructing DNA replication in vitro (10, 11). It has also been observed that intact Novikoff ascites hepatoma cells exposed to potassium chromate formed cross-linking of nuclear proteins to DNA (12). Recently, Cr(III) was shown to cause mutation...

The interaction of calf thymus DNA with Cu2+ and Pb2+ was studied in aqueous solution at pH 6.5 with metal/DNA (P) (P = phosphate) molar ratios (r) 1/80, 1/40, 1/20, 1/10, 1/4, 1/2, and 1, using Fourier Transform ir (FTIR) spectroscopy. Correlations between the ir spectral changes, metal ion binding mode, DNA condensation, and denaturation, as well as conformational features, were established. Spectroscopic evidence has shown that at low metal/DNA (P) molar ratios 1/80 and 1/40, copper and lead ions bind mainly to the PO2- of the backbone, resulting in increased base-stacking interaction and duplex stability. The major copper ion base binding via G-C base pairs begins at r > 1/40, while the lead ion base binding occurs at r > 1/20 with the A-T base pairs. The denaturation of DNA begins at r = 1/10 and continues up to r = 1/2 in the presence of copper ions, whereas a partial destabilization of the helical structure was observed for the lead ion at high metal ion concentration (r = 1/2). Metal-DNA binding also results in DNA condensation. No major departure from the B-family structure was observed, upon DNA interaction with these metal ions.

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The Interaction of calf-thymus DNA with aspirin is Investigated In aqueous solution at pH 7-6 with drug/DNA(phospbate) molar ratios of r = 1/40, 1/20, III0, 115, 112, I and 2. Fourier transform Infrared (FTIR) and laser Raman difference spectroscopy are used to determine drug binding sites, sequence preference and DNA secondary structure, as well as the structural variations of asplrin-DNA complexes in aqueous solution. Spectroscopic evidence showed that at low aspirin concentration (r = 1/40), drug-DNA interaction is mainly throngh the backbone PO~ groups and the A.T base pairs. Such intentetlon larply perturbs the phosphate vibration at 1227, cm -I end the A-T bands at 1663 and 1609 em -t with no major helix des~bllizatlon. At higher drug concentration (r > 1120), the participation of the G-C bases in dmg.DNA complexation was evident by strong perturbations of the guanine and cytosine vibratious at 1717 and 1494 em -t, with a partial helix destubilLmtion. A major alteration of the B-DNA structure towards A-DNA occurs on drug eomplexation. The aspirin interaction was through anion CO and COOCHs donor atoms with those of the backbone PO2 group and DNA bases donor sites (directly or indirectly via I"!20 molecules). cificity, helical stability and DNA secondary structure in aqueous solution. Vibrational spectroscopy (Raman and infrared) is often used to characterize the nature of drug-DNA interaction and to monitor the effects of various drugs on DNA structure [7][8][9]. Recently, we applied vibrational spectroscopy to analyse the nature of vitamin C complexatioa with calf-thymus DNA at physiological pH and different drug concentrations [I0]. This study allowed us to determine the drug binding sites, the sequence specificity, the DNA secondary structure and the helical stability, in aqueous solution. We believe that vibrational spectroscopy can also be used here, in order to investigate the complex formation between DNA and aspirin and to elucidate the nature of this biologically important drug-DNA interaction.In this communication, we report the interaction of calfthymus DNA with aspirin in water/methanol (75:25, v/v) solution at pH 7-6 with aspirinlDNA(P) molar ratios r = II 40 to 2, using FTIR and laser Raman spectroscopic techniques, that have not been previously reported.

The involvement of the Fe cations in autoxidation in cells and tissues is well documented. DNA is a major target in such reaction, and can chelate Fe cation in many ways. The present study was designed to examine the interaction of calf-thymus DNA with Fe(II) and Fe(III), in aqueous solution at pH 6.5 with cation/DNA (P) (P = phosphate) molar ratios (r) of 1:160 to 1:2. Capillary electrophoresis and Fourier transform infrared (FTIR) difference spectroscopic methods were used to determine the cation binding site, the binding constant, helix stability and DNA conformation in Fe-DNA complexes. Structural analysis showed that at low cation concentration (r = 1/80 and 1/40), Fe(II) binds DNA through guanine N-7 and the backbone PO(2) group with specific binding constants of K(G) = 5.40 x 10(4) M(1) and K(P) = 2.40 x 10(4) M(1). At higher cation content, Fe(II) bindings to adenine N-7 and thymine O-2 are included. The Fe(III) cation shows stronger interaction with DNA bases and the backbone phosphate group. At low cation concentration (r = 1:80), Fe(III) binds mainly to the backbone phosphate group, while at higher metal ion content, cation binding to both guanine N-7 atom and the backbone phosphate group is prevailing with specific binding constants of K(G) = 1.36 x 10(5) M(-1) and K(P) = 5.50 x 10(4) M(-1). At r = 1:10, Fe(II) binding causes a minor helix destabilization, whereas Fe(III) induces DNA condensation. No major DNA conformational changes occurred upon iron complexation and DNA remains in the B-family structure.